1.Electrical properties
Volume resistivity, DC breakdown strength, dielectric loss tangent, and dielectric constant.
1. Volume resistivity The volume resistivity ρν of an insulating material refers to the ratio of the DC electric field strength in the direction of the volume current of the sample to the current density at that location. ρν=EV/jv(Ωcm), where EV is the DC electric field strength, and jv is the current density. 95% alumina ceramic is an excellent electronic insulating material with high volume resistivity. It is stipulated in the national standard GB/T5593-1999 that at 100°C, ρν≥1×1013Ωcm; at 300°C, ρν≥1×1010Ωcm; At 500℃, ρν≥1×108Ωcm. In fact, the volume resistivity of 95 porcelain produced in my country is 1-2 orders of magnitude higher than the above regulations. Instruments for measuring volume resistance usually use a high resistance meter.
2. DC breakdown strength The DC breakdown strength of electrical insulating materials refers to the changes that occur under the action of an external DC electric field, mainly caused by changes in the internal structure. When the electric field strength reaches a certain value, further changes in its internal structure will be promoted, and insulation breakdown will occur. The national standard GB/T5593-1999 stipulates that the withstand voltage test of 95% alumina ceramics is carried out in the case of DC. When a DC voltage is applied to the sample, the sample breaks down, and the breakdown voltage value is related to the average thickness of the sample. The ratio is called DC breakdown strength, unit: KV/mm. The national standard GB/T5593-1999 stipulates that it should be greater than 18KV/mm. In fact, we can generally reach 30-40KV/mm.
3. Dielectric constant It is a parameter of dielectric polarization degree of insulating material under AC electric field. It is the ratio of the capacitance of a capacitor filled with a certain insulating material to that of a capacitor with the same electrode size when a vacuum is used as the medium. It represents an inherent property of the material. The national standard stipulates that when the test frequency is 1MHz, the dielectric constant of 95% alumina ceramics is between 9-10.
4. Tangent value of dielectric loss Dielectric loss means that under the action of AC electric field, the material undergoes polarization or absorption phenomenon, resulting in power loss, and usually there is a phenomenon of heating on the dielectric material. The magnitude of the dielectric loss is represented by the tangent of the dielectric loss angle. The national standard GB/T5593-1999 stipulates that when the frequency is 1MHz, 95% alumina ceramics are required to reach 4×10-4.
2. Thermal performance
1. The average linear expansion coefficient The relative elongation of ceramics when the unit temperature is raised, that is, the average linear expansion coefficient is one of the main performance indicators of 95% alumina ceramics. The national standard GB/T5593-1999 stipulates that the test temperature is 20-500℃, and a is 6.5×10-6-7.5×10-6/℃; when the test temperature is 20-800℃, a is 6.5×10-6- 8×10-6/℃. Since ceramic parts are generally sealed with metal parts, their expansion coefficients are required to match those of metal materials.
2. Thermal shock resistance refers to the ability of ceramic materials to withstand rapid cooling and rapid heating. 95% alumina ceramics have good thermal shock resistance and can withstand the test of high temperature and rapid cooling and heating without damage. The national standard GB/T5593-1999 stipulates: Put 10 ceramic samples into a heating furnace at 800±10℃, keep warm for 30 minutes, take them out and place them on asbestos plates, cool them to room temperature naturally, and then put the samples into the furnace. In a heating furnace at 800°C, according to the above regulations, carry out a total of 10 times, and finally immerse it in a 1% concentration of magenta solution for 3 minutes, take it out, wash it with tap water, wipe it dry, and observe the sample under the light. There are no cracks or bursts in the samples, and the performance is qualified.
3. Mechanical properties
Mechanical properties refer to the ability of a material to withstand mechanical force without breaking. Usually, the tensile strength, flexural strength and compressive strength are different according to the direction of the force and the point of force.
The tensile strength of 95% alumina ceramics is generally above 160Mpa, which is usually greater than the best sealing strength. There are no technical indicators for the tensile strength and compressive strength of 95% alumina ceramics in the national standard GB/T5593-1999. The national standard GB/T5593-1999 stipulates that the flexural strength of 95% alumina ceramics is 280MPA.
4. Other properties
1. Bulk density The bulk density of 95% alumina ceramics has a great relationship with the selection of raw materials and the process of making ceramics. In the national standard GB/T5593-1999, the bulk density of 95% alumina ceramics is required to be above 3.60g/cm3. In fact, the density of 95% alumina ceramics varies greatly depending on the molding method of porcelain making. Usually, the density is between 3.60-3.70 g/cm3 during hot die casting and grouting; and 3.70 g/cm3 during isostatic pressing. cm3; gel forming can reach 3.73 g/cm3.
2. Air tightness 95% alumina ceramics are used in microwave devices, vacuum switch tubes, etc., and require high vacuum air tightness. In the electronic industry standard SJ/T11246, it is stipulated that the air leakage rate of the sealing part QK≤1×10--11Pa·M3/S. In fact, it is stipulated that the air leakage rate of the ceramic itself should also meet the above standards.
3. Chemical stability The chemical stability of 95% alumina ceramics is characterized by its ability to resist corrosion by various chemical reagents. The ability to resist corrosion by acid reagents is called acid resistance, and the ability to resist corrosion by alkaline agents is called alkali resistance. The national standard GB/T5593-1999 stipulates that the sample is placed in a 1:9 hydrochloric acid or 10% sodium hydroxide solution, and the solution is heated to 100 ° C and kept for 1 h. Acid resistance requires Kh≤7mg/cm2, and alkali resistance requires Kn≤0.2mg/cm2.
4. Microstructure of ceramics The microstructure of ceramics refers to the composition, content, distribution, morphology, particle size distribution, uniformity, defects, interphase of crystal phase (main crystal phase, secondary crystal phase), glass phase, gas phase, and grain boundary. The inter-combination relationship of matter, domain structure, etc. The microstructure is closely related to the process of making ceramics, which directly affects the performance of ceramics. It is a research method to improve the quality of ceramics and improve the process.
5. Sealing performance of 95% alumina ceramic-metal
As a class of structural ceramics, 95% alumina ceramics are to be sealed with metals to form a component. Especially the 95% alumina ceramics used in microwave devices and vacuum development tubes usually take the ceramic-metal sealing performance as an important index to evaluate. The content of the assessment: the ceramic-metal sealing performance requires high strength and good air tightness. It is stipulated in the electronic industry standard SJ/T11246 "Ceramic Shells for Vacuum Switch Tubes" that the average sealing strength is σa≥90Mpa when using standard ceramic sealing; when using the three-point test, σa≥90Mpa. The air tightness of the seal, the helium leak rate Qk≤1×10-11Pam3/s
